In order to control current of an X-ray tube, heating current of a cathode (e.g., an emitter) of the X-ray tube is varied. As a result, the electron emission of the cathode is controlled. Such control is described in the published patent application DE 43 00 825 A1.
In the case of pulsed radiography or in the case of 3D imaging in angiography, a dose power of X-ray radiation and thus a required tube current from pulse to pulse are adapted to a present object situation. For this purpose, an X-ray generator that heats the cathode to an emission temperature receives corresponding desired value stipulations for the tube current in each case shortly before the X-ray pulses. By virtue of emission tables stored in the X-ray generator, this results in an appropriate start value of the heating current. However, since typical pulse widths are only in the range of 3 to 12 ms, control only during the radiation pulses is ruled out. In the pulse pauses, too, the emitter is to be controlled to the required temperature for reasons of the desired function. Typical image frequencies are in the range of 3 to 100 Hz.
It is known, in the case of a desired value jump in the tube current, to select the corresponding heating current with the aid of an emission table and to adjust the heating current by a heating current controller that is responsible for controlling the heating current in the pulse pauses. On account of the thermal delay of the emitter, the tube current follows only in a time-delayed manner.
A tube current controller is active during a pulse, and the tube current controller provides feedback about the present emitter temperature directly via the measurement of the tube current in the high-voltage circuit. The tube current controller may attempt to bring the emitter to the required temperature as rapidly as possible by a large control dynamic range within the allowed limits for the heating current. This functions very effectively in the case of long X-ray pulses (e.g., greater than 10 ms), but in the case of short pulses (e.g., 3 ms), the influencing possibility for the tube current controller decreases greatly owing to the short control time. Consequently, owing to a lack of emission feedback, the more sluggish heating current controller dominates the emitter temperature or the rate of adjustment thereof. Faster tube current adjustment times are to be provided, however, owing to faster rotation times, higher image frequencies and shorter pulse times in the case of 3D imaging.
FIG. 1 shows a block diagram of known heating current control of one embodiment of an X-ray tube 1 operating in pulsed operation. In this case, according to the design, during the pulse pauses, the heating current H is controlled based on stored emission tables with the aid of the measured actual value of the heating current HActual, and during the X-ray pulses, the heating current H is controlled with the aid of the measured actual value of the tube current RActual. The X-ray tube 1 includes at least one emitter 2 and at least one anode 3. The at least one emitter 2 is supplied with the heating current H from the controllable current supply 4. The tube voltage is generated by a controllable tube high-voltage supply 5.
A heating current measuring unit 6 is situated in the heating current circuit and determines the actual value of the heating current HActual. The actual value of the heating current HActual is fed to a heating current control unit 8. A tube current measuring unit 7 is situated in the tube voltage circuit and determines the actual value of the tube current RActual. The actual value of the tube current RActual is likewise fed to the heating current control unit 8. The actual values HActual and RActual are compared with the desired value of the heating current HDesired in the pulse pauses and with the desired value of the tube current RDesired during the pulses in the heating current control unit 8. A controlled variable RG is derived therefrom as necessary and controls the current supply 4. The heating current control unit 8 is embodied as a PI controller, for example.
FIG. 2 shows a timing diagram of relevant variables of the heating current control. This diagram is appropriate with respect to the heating current control in FIG. 1. The control during the pulses is not illustrated for reasons of clarity. Such additional control would become apparent as heating current peaks during the pulses and adjust the tube current to the desired value somewhat more rapidly.
The illustration shows on the x-axis the time t in milliseconds, and on the y-axis merely phenomenologically (without indications of magnitude) the actual value of the tube current RActual, the desired value of the tube current RDesired, the temperature T of the emitter 2 and the actual value of the heating current HActual. Owing to a material-governed delayed heating of the emitter 2, the temperature T does not rise abruptly, but rather in a time-delayed manner. As a result, the actual value of the tube current RActual attains the desired value RDesired only after approximately 500 ms. In the example illustrated, the pulse widths and pulse pauses in each case have a length of approximately 75 ms.
A dynamic correction that adapts the heating current H in the pulse pauses if the tube current R deviates from the nominal value during a pulse would accelerate the adjustment, but also leads to overshoots and undershoots beyond the actual desired operating point. In the case of limit-load scans, this may lead to an overload of the X-ray tube and thus to arcing. This causes unusable 3D scans and may also lead to damage to the X-ray emitter.